With the continuing increase in the living standards of the people, there is also an increasing demand on the market for foods and beverages, and the production of food and beverage cans has been growing each year. Severe competition in the metal packaging industry for food and beverages has arisen in this tide of economic development. For this reason, in order to conserve natural resources, lower costs and meet the demands of market competition, research on the saving of materials and the development of gas pressure-resistant metal pop-top covers is not only a necessity for the survival and development of the industry, but it is also essential for the economic development of the market.
At present, most of the metal beverage containers on the market make use of gas pressure-resistant metal pop-top covers that have small openings. This type of pop-top cover is general comprised of two components, a lid and a pull-ring. The pull-ring is riveted to the lid. However, in order for the pop-top to have the characteristic of gas pressure-resistance, the main body of the lid, which begins at the lid's circumferential edge and extends toward the center, is designed with a concave countersink structure. When beverage cans that use metal pop-top covers are subjected to increased internal pressure (for example, because of temperature increase), the lid may become unstable and slip, with the result that the internal volume of the can is increased in order to prevent danger from arising due to excessive internal pressure. For a long time, in order to maintain high pressure resistance strength on the part of the lid in the face of buckling due to destabilization, the countersink inclination has been designed to be comparatively small, with a course of inclination of 1-14° as shown, for example, by angle A in FIG. 1. However, as the demand for savings on materials continually increases, the question of how to continually decrease the notch diameter of the material and its thickness has become a very important topic.